A surge arrester is used to protect relatively expensive electrical equipment from damage during periods of over-voltage in which the voltage to which the electrical equipment is exposed is higher than a normal operating range. The surge arrester diverts current around the electrical equipment to ground during periods of over-voltage, thereby shielding the electrical equipment from the high voltages and corresponding currents. Prolonged exposure to abnormally high voltages may cause the surge arrester to fail in a short-circuited state.
If no mechanism is provided for disconnecting the failed arrester from the circuit, the arrester is said to have failed closed. After failing closed, the surge arrester prevents current from flowing to the electrical equipment even after the period of over-voltage, which prevents normal operation of the electrical equipment. If a mechanism is provided for disconnecting the failed arrester from the circuit, the arrester is said to have failed open, in which case the electrical equipment may operate normally. However, the electrical equipment that was protected by the surge arrester that has failed open is no longer protected by the surge arrester.
Conventional surge arresters include one or more metal oxide varistor (MOV) disks that are held in compression within a fiberglass filament wound tube between a fixed snap-ring electrode and a removable spider spring assembly. Current flows through the electrode and the MOV disks during periods of over voltage and when the surge arrester has failed closed. The spider spring assembly is a mechanical device that is expelled out of the end of the tube in the event of arrester failure. The spring force on the MOV disks is consequently released, and the electrode and the MOV disks drop out of the tube, thereby breaking the electrical pathway through the surge arrester and permitting current to flow to the electrical equipment. In some cases, the tube may be projected upward in response to the release of the spring force. Machining required to create venting slots through the side wall of the tube and to cut grooves into the tube to accept the electrode and spider spring assembly may be expensive.
To enable the venting of gases generated within the filament wound tube, pre-formed weaknesses may be created in the side wall of the tube. Pre-formed weaknesses are areas of the tube where the side wall is thinner than usual. When the pressure of the gases within the tube exceeds a maximum pressure that may be withstood by the pre-formed weaknesses, the planned weaknesses break to provide paths through which the gases may be vented. Pre-formed weaknesses are necessary because the walls of the tube are otherwise too thick, and the pressure required to break through the walls is too great. Machining the pre-formed weaknesses into the filament wound tube may be expensive.